The water surrounding Madling Isle festers with life, most of it extremely dangerous, from a vast array of Earth’s earlier time periods. Megalodon, merfolk, giant sea scorpions, ichthyosaurs, orca and an impossible host of other creatures inhabit the waters below the perpetually rough waters.
The most feared of all these creatures is the giant possibly immortal liopleurodon, Dreadfin. In B/X terms Dreadfin functions exactly as a 12 HD Sea Dragon (Expert book pg 39) with a Morale of 10. A natural 20 on the liopleurodon’s attack roll results in a medium sized creature being swallowed whole, or alternately the utter destruction of all but the most fortified watercraft.
I was tempted to do a bunch of new monsters, but the Madling Sea provides ample opportunity to use a ton of very underused critters- whatever your preferred edition.